Field of the Invention
The present invention relates in general to a private automatic branch exchange (referred to hereinafter as PABX) for an integrated services digital network (referred to hereinafter as ISDN), and more particularly to a primary rate interface (referred to hereinafter as PRI) apparatus for the PABX.
Description of the Prior Art
Currently, digital communication systems are generally classified into North America and Europe types. The digital communication system of the Europe type provides 64 Kbps as a basic speed of an ISDN and performs line coding using a high density bipolar 3 (referred to hereinafter as HDB3) manner to minimize an error generation probability. In order to implement a PABX for the ISDN in the above-mentioned manner, a digital trunk interface apparatus has been required to provide a PRI channel.
As an example of ISDN techniques, a line switching apparatus for an ISDN data transmission system is disclosed in Japanese Patent Laid-open publication No. Heisei 4-290032, and is shown in a block form in FIG. 1, herein. As shown in FIG. 1, the line switching apparatus for the ISDN data transmission system comprises a line interface switching device 6a with a terminal adapter 61a and a line interface switching device 6b with a terminal adapter 61b. The terminal adapter 61a of the line interface switching device 6a performs a switching operation to interface a terminal controller 7 or a terminal branch device 8a to an ISDN 2 upon generation of line trouble. The terminal adapter 61b of the line interface switching device 6b performs a switching operation to interface a terminal branch device 8b to the ISDN 2 upon generation of the line trouble. The above-mentioned conventional line switching apparatus is desirable to perform a backup operation against the line trouble, but has the disadvantage that it cannot provide a direct interface between subscriber boards.